Somatic hypermutation of variable (V) genes, which encode a portion of immunoglobulin molecules, occurs at a frequency that is a million times greater than mutation in other genes. The molecular mechanism that introduces these mutations is unknown. The project has two major aims. The first goal was to determine which DNA polymerases are expressed in B lymphocytes undergoing mutation. DNA polymerases alpha, beta, delta, epsilon and zeta are expressed in the rapidly dividing cells from germinal centers, and they could all be candidates for the enzyme introducing mutations. However, polymerases alpha and epsilon are not expressed by B cells outside of germinal centers, suggesting that these cells are resting and are deficient for DNA repair that utilizes polymerase epsilon. In addition, disruption of polymerase zeta in a mouse model caused embryonic lethality, suggesting that it is essential for cell viability during mammalian embryonic development. The second goal was to analyze hypermutation in V genes from old and young humans to determine if the frequency or pattern of mutation changes with age. The frequency of mutation was identical in both groups, indicating that old humans have hypermutated antibodies with high affinity for antigens. The frequency of tandem mutations, which have been associated with defective DNA mismatch repair in mice, varied among individuals. Microsatellite variability in DNA, which is caused by impaired mismatch repair, was then measured, and there was a strong correlation between the frequency of tandem mutations and microsatellite alterations. The data suggest that tandem mutations in human variable genes are related to defective mismatch repair.